1. Field of the Invention
The invention relates to a thermo-optical switch having a layer structure on a substrate and containing, in a waveguide layer, a directional coupler waveguide structure and, above the waveguide layer, a heating electrode configured to complement the form of the coupler structure.
For the transmission of broadband optical signals without prior conversion into electrical signals, it is necessary to utilize cross-connects which may be switched to a state of optical transparency. Such optically transparent switches contain, among others, spatial switches for directing incoming optical signals to selected output fibers. The spatial switches must satisfy the following requirements: low cross-talk, low coupling attenuation, independence of signal polarization, low electric switching power, response times &lt;10 ms. high integration density, low production costs.
2. The Prior Art
In recent years, thermo-optical switches have been developed on a polymer basis because the properties of polymeric waveguides give rise to the expectation that the above-mentioned requirements may be realized with them by way of selective structuring. Thus, polymers have a large thermo-optical coefficient, i.e. a change in temperature causes a large change of their refractive index, combined with low thermal conductivity. These properties result in low switching power for a thermo-optical switch which is below that of a comparable SiO.sub.2 switch by a factor of about 100. Since polymers display very low birefringence they can be used for the fabrication of components which are independent of polarization. Switching times are in the range of milliseconds, 1 to 10 ms being typical.
Moreover, the use of polymer waveguides makes it possible to fabricate spatial switches by relatively simple processes which are well-known from the fabrication of microelectronic components. In addition, polymer technology makes it possible to integrate on a single substrate, as hybrid technology, a plurality of optical components, such as, for instance, III-V-lasers, photo diodes with polymer waveguides, networks and switches. Thus, components with complex functions may be fabricated in a cost-efficient manner.
Proceeding from the above-mentioned state of knowledge, solutions have been sought in recent years, to utilize as many of the above-mentioned advantages of polymers for optical elements as possible. Since the necessary switching power and switching time of thermo-optical elements are primarily dependent upon their thermal properties, i.e., their thermal conductivity, thermo-optical coefficient and the heat capacity of waveguide layer, buffer layers and substrate material, as well as upon the shape and size (dimensioning) of the waveguides and heating electrode, there are known in the art many thermo-optical elements differing in their concrete structure for optimally realizing defined functions.
In Journal of Lightwave Technology, Vol. 7, No. 3 (1989), pp. 449-453, there is described a planar thermo-optical switch in which a polymeric waveguide layer made of polyurethane is arranged upon a PMMA (polymethyl methacrylate) substrate, with a PMMA buffer layer superimposed thereon on which is provided a silver strip conducting electrode as a heating element. At a switching power of 100 mW, typical switching times are 12 ms for on-off switching and 60 ms for off-on switching.
In most thermo-optical switches based on polymer, the waveguides are formed in strips which leads to reduced switching times and switching power. Thus, a digital optical switch (DOS) which is independent of polarization is described in SPIE, Vol. 1560, Nonlinear Optical Properties of Organic Materials IV (1991), pp. 426-433 in which a gold strip electrode is provided on one of the two output branches of a symmetrically structured Y-junction. When a heating voltage is applied to the electrode it realizes an asymmetric effect upon the described switch. The change of the refractive index of the amorphous polymeric material of the waveguide is generally isotropic and, therefore, independent of any polarization as regards light propagating through the structure. The monomodal waveguide made of DANS polymer arranged upon a glass substrate was fabricated by photo bleaching the non-waveguiding areas of the waveguide layer with UV irradiation and is covered by a buffer layer. The switching times of this arrangement are in the millisecond range.
In Proc. 21st Eur. Conf. on Opt. Comm. (ECOC '95--Brussels), pp. 1063-1066, there is also described a Y-shaped waveguide in a polymer-based digital optical switch. In this case the waveguide structure has been realized by photo lithography followed by dry-etching of moats in a silicon substrate, followed by thermal oxidation in water vapor and, thus, creation of a SiO.sub.2 buffer layer, spinning of CYCLOTENE.RTM. polymer thereon and covering the polymer layer by a further SiO.sub.2 buffer layer. A titanium thin film electrode is divided and positioned above the two output branches. At a switching power of between 130 mW and 230 mW the extinction coefficient in the heated branch is better than 20 dB. The optical power is then fed wholly through the other--unheated--branch. But even in this technical solution the requisite switching power and switching times are still too high.
In European Patent EP 0,642,052 there is described a polymer-based digital optical switch in a layer structure consisting of a substrate, lower buffer layer, waveguide layer, upper buffer layer and heating element with a Y-shaped waveguide structure, wherein the refractive indices of the two buffer layers are lower than the refractive index of the waveguide layer. Moreover, the refractive index of the buffer layer adjacent to the heating element is lower than that of the lower buffer layer. Ranges covering the contrast of the refractive indices have been disclosed in accordance with parameters desired (optical loss, switching power) for realizing a predetermined function, and the dimensions of the output branches of the waveguide are structured symmetrically or asymmetrically, and the heating elements are also arranged symmetrically (at both output branches) or asymmetrically (at one output branch only). While precise current control is not required for the described arrangement it requires a higher switching power and leads to cross-talk of not more than about -20 dB.
Low switching power is required in a thermo-optical switch described in IEEE Photonics Technology Letters, Vol. 5, July 1993, pp. 782-784. The switch is provided with a Mach-Zehnder-interferometer above the two waveguides of which there are arranged thin-film heating elements. While this optical switch does realize low cross-talk as well, its overall length is about thrice that of a conventional directional coupler.
In connection with a different system of waveguide material there is described in Electronics Letters, Oct. 29, 1981, Vol. 17, No. 22, pp. 842-843, a thermo-optically induced waveguide based upon LiNbO.sub.3 :Ti in which a nickel-chromium electrode is arranged on one section of the waveguide. When a voltage is applied to the electrode the refractive index of the area of the waveguide below the electrode changes thus deflecting the fed-in light.
Also, directional coupler switches with alternating .DELTA..beta. are known, as described, for instance, in IEEE Journal of Quantum Electronics, Vol. QE-12, No. 7, pp. 396-401, July 1976. In this case, several electrode sections are arranged upon parallel waveguides made from the previously mentioned LiNbO.sub.3 material. In actual switching conditions the electrodes generate in the corresponding waveguide sections below them, based upon the electro-optical effect, a difference in the propagation velocities of the light of respective alternating signs. If the interactive length between the two waveguides is greater than the coupling length the desired switching state (cross-over or throughput state) may be set by way of the switching power.
A directional coupler with alternating .DELTA..beta. is also described in Patent Abstracts of Japan, Vol. 12, No. 192 (P-712), Jun. 4, 1988 #JP 62.297,827 (Fujitsu Ltd.), in which in a first variant several (here two) electrode sections are arranged congruently over each of two parallel waveguides, and in a second variant the electrode configuration is structured in a step-like manner whereby each of the horizontal electrode sections covers about half of the coupling length of the waveguides and wherein the areas of the waveguides covered by the electrode sections are displaced relative to each other. With such a directional coupler it is possible, proceeding from a cross-over state which for technical reasons is bad in terms of realizing a cross-over/throughput switching function, only to switch in the throughput direction (symmetric switch).
In connection with testing and calculating the coupling properties of directional couplers formed by strips of a dielectric material on a LiNbO.sub.3 substrate and metal films between those strips, there are described in APPLIED OPTICS, Vol. 17, No. 5, Mar. 1, 1978, pp. 769-773 different possibilities of the placement of the two waveguides in which the coupling coefficient is never constant.
The state of the art upon which the invention is based is described in several publications all of which describe the same subject: OFC '95, Postdeadline Papers, PD 17-1, 1995; MICRO SYSTEM Technologies '94, 4th Int. Conf. on Micro, Electro, Opto, Mechanical Systems and Components, Berlin, Oct. 19-21, 1994, VDE-Verlag GmbH., pp. 1097-1100; Jahresbericht 1994 des Heinrich-Hertz-Instituts fur Nachrichtentechnik Berlin GmbH., pp. 54-55; SPIE Proceedings Series Vol. 2449, 1994, pp. 281-292 may be mentioned. In the last-mentioned publications there is described a thermo-optical tuneable (4.times.4) switching field fabricated in polymer technology in an integrated optical form, the basic element of which is a controlled thermo-optical switch of the kind mentioned above, structured as a 2.times.2 directional coupler.
This 2.times.2 directional coupler is provided with two symmetrically arranged waveguides the center portions of which are spaced closely to each other so that under controlled conditions there will be cross-talk of light from one waveguide into the other one. When a voltage is applied to it, the electrode which is positioned over one waveguide only will heat this waveguide somewhat so that its refractive index changes affecting a transfer of light from one of the waveguides into the other one. The heat generated in the heating electrode diffuses through the upper buffer layer, the waveguide layer and the lower buffer layer into the silicon substrate which acts as a heat sink. Owing to the negative temperature coefficient of the waveguide, this leads to a change of the refractive index in the waveguide and of the propagation coefficient of the waveguide. As has already been mentioned, the effect of the thermo-optically induced phase shift in waveguides is used for switching in Mach-Zehnder or directional coupling structures. The asymmetric coupler is very short and consumes little power. The extinction ratio in the initial cross-over state is set by the selection of an appropriate coupling length; a subsequent adjustment is not possible. Process-conditioned fabrication tolerances limit the extinction ratio in the cross-over state to typically -25 dB which results in minimal cross-talk of only -21.5 dB in the 4.times.4 matrix. In different coupling elements with an electrode length of 3 mm extinction ratios were measured between 20 dB (cross) and 32 dB (straight), at a power consumption of 30 to 40 mW. The switching times were stated to be less than 1 ms. The coupler is structured to be in its cross-over state when the electrode is not heated, i.e. light coupled into one of the input gates is coupled from the input waveguide into the parallel adjacent waveguide and exits at the output thereof. When the electrode is heated, the light exits at the output gate of the same waveguide ("bar").
The described switching arrangements are processed under dust-free conditions. To this end, a silicon substrate, which also serves as a heat sink, is covered with an SiO.sub.2 passivation layer by thermal oxidation. Thereafter, the PMMA waveguide layer and a further passivation layer made of Teflon.RTM. are applied in succession by a spin process. The PMMA is doped with a photo initiator molecule in which a photochemical process is released (light induced photo locking) under intense exposure to UV radiation, which leads to an increase of the refractive index of the waveguide layer. The integrated optical light waveguides of a width of a few pm are defined by localized UV exposure through a photo mask. The refractive index and the difference in refractive indices between the exposed and unexposed areas may be set very precisely over a wide range by selecting appropriate mixture ratios of photo initiator and PMMA and by varying the dose of exposure. In a subsequent process step the remaining photo initiator molecules are removed from the unexposed areas of the waveguide layer by heating, and the waveguide structures will thus be fixed. As a final step, an aluminum-gold layer is vapor deposited, from which the micro heating electrodes are etched by wet chemical action.
Aside from the electrode configuration which is asymmetric relative to the optical axis described in respect of an actual switch, a symmetric electrode configuration has also been mentioned in which a unitary strip-shaped heating electrode is arranged symmetrically relative to the optical axis of the directional coupler. Hence, both waveguides are affected identically by the heating electrode so that simultaneous coupling may occur between the overlapping modal ends. Therefore, a transfer of 100% of the optical power from one waveguide into the other one of the symmetric waveguide configuration and, accordingly, a high extinction ratio may in principle be achieved. However, the switching power required for operating the symmetrical switch is too high (for polymer waveguides several hundred mW per switch), so that they have not been used in practice. Even the last-mentioned arrangements suffer from excessive power consumption and cross-talk.